Many speech coder families are already known, such as the vocoder and Linear Prediction Coder (LPC) families. The vocoder family derives the original speech signal from a set of coefficients used to process the original speech signal and derive therefrom a residual signal. A pitch information is then derived from the residual for voiced speech sections, otherwise the residual signal is simply made to be noise. The correlative decoding process involves modulating back a synthesized pitch or noise signal by the coefficients. The relative efficiency (quality versus bit rate) of such a coding scheme is rather poor unless performing a very precise determination of the pitch value. This already shows the significance of any efficient method for determining the pitch. Also with a reasonable increase in the complexity of the coder, the LPC coder family provides valuable improvement to the coding/decoding operation. Needless to mention the importance of any savings into the bit coding rate and or the coder complexity, for the voice processing industry. Saving in computing complexity enables minimization of processor workload, while saving in bit rate is of major importance in voice transmission or in storage facilities. These reasons enable understanding the full meaning of engineers efforts to optimize their coders in order to save a few coding bits, i.e. minimize the bit rate required for coding the speech signal, while keeping the coding quality quite unchanged.
The above considerations not only enable appreciating the engineering value of one coding scheme versus the others, but they might be of great significance to business value appreciation of a given coding/compressing scheme.
In summary, in the LPC type of coding schemes one may improve the coding/decoding quality considerably by efficiently detecting the pitch and by adding more information than usually done about the residual signal. Significant improvements are made by judiciously designing the coder even within a same sub-family of coders such as the ones known as:
Voice Excited Predictive Coder (VEPC) as disclosed in IBM Journal of Research and Development Vol. 29, Number 2, March 1985;
Multi-Pulse Excited Coder (MPE); or
Regular Pulse Excited Coder (RPE), as disclosed in the article "Regular Pulse Excitation, a Novel Approach to effective and Efficient Multipulse Coding a Speech", published by P. Kroon et al. in IEEE Transactions on Acoustics Speech and Signal Processing Vol ASSP 34 N05 Oct. 1986; and in a Thesis "Etude, Simulation et mise en oeuvre sur microprocesseur de codeurs predictifs multiimpulsionnels", presented by E. Landon, on Nov. 22, 1985 before the University of Nice, France.